Modern medical diagnostic methods with radioactive substances rely on proper detection of emitted radiation or its absorption within a patient to whom such radioactive substances have been administered. Such detection is typically carried out with gamma-cameras or comparable devices.
Appropriately accurate detection in turn requires a constant quality testing and monitoring of the detection device or the gamma-camera itself. More in detail, the gamma-camera or detection device must allow for a homogeneous detection of emitted radioactivity over its entire surface. Inhomogenities in the detection itself will spoil or reduce the value of measurements in patients for diagnostic purposes.
Assessing quality and homogenity of a detection device is presently carried out with so-called “flood sources,” or “radiation flood sources”. These flood sources are large area (>10×10 cm2) radiation sources, predominantly in form of flat foils or sheets. One example of a commercially available flood source is a plastic matrix in form of a circular flat sheet (diameter >30 cm) which contains the radionuclide of choice within the plastic sheet. However, these plastic matrices into which the respective nuclide or mixture of nuclides is incorporated typically do not allow for providing of a homogeneous radiation field, when analysed with a gamma-camera. Thus, the present technology of providing a flood source in form of a plastic sheet into which nuclides are incorporated requires extensive testing and scanning e.g. by use of photographic films as quality control of the flood source itself. Typically such product still provides for deviations of about 10% in the emitted radiation over its surface. Thus, the present products cannot exactly provide for a homogeneous radiation field.
Various methods have been proposed to achieve an improved radiation flood source. In particular evaporation and vapor phase deposition by distillation of some organic compounds has been proposed. The equipment required is somewhat complicated in that a vacuum system is necessary. It also has its limitations in the possibility of obtaining suitable organic compounds. The precipitation technique is very versatile for carrier quantities above 1 mg. The self-scattering and self-absorption factor is, however, large and in many cases difficult to estimate because filtered and dried samples tend to become quite uneven.
In cases where the amount of carrier is small (<1 mg) evaporation is a common method for source preparation. Samples prepared by this method often become very unevenly spread, however, with most of the material forming a ring. Addition of insulin, colloidal silica, or cupric ferrocyanide to the drop before evaporation improves the samples, but neither of these methods is satisfactory. Besides the addition of some solid compound is required, which will of course increase the self-scattering and self-absorption factor.
A further method which has been proposed in literature is the electrospraying of radionuclides (see E. Brunix and G. Rudstam in: Nuclear Instruments and Methods 13 (1961) p. 131–140). This method requires an electrically conducting substrate and electrically charged nuclides. The nuclides migrate between a first electrode in solution through the nozzle of a pipette to the substrate, functioning as a second electrode. Typically the electrospraying therefore has the disadvantage that only electrically conducing substrates can be coated, the coated area is small and the equipment used is very expensive.
Another method of depositing a radioactive coating on a substrate, in this case a brachytherapy device is disclosed in WO 99/62 074. The document teaches printing of a radioactive fluid by use of the conventional inkjet technology and curing the printing fluid to obtain the final coating. Due to the intended substrate of a brachytherapy device or seed, the printing process disclosed in WO 99/62 074 is concerned with small dimensions.
The printing fluid used in the above process is stated to comprise the nuclide, preferably selected from Pd-103, I-125, Au-198, Au-199, Y-90, P-32, Ir-192, and Am 241, or a precursor thereof, a solvent and a curable binder to retain the nuclide within the coating. The printing fluid, which is deliberately not called an ink, is not intended to be viewed as a black or coloured ink and all corresponding examples lack an ink as a possible constituent of the printing fluid. To nevertheless allow for printing of the fluid WO 99/62 074 teaches that specific requirements (viscosity, charge etc.) must be met by the printing fluid disclosed. This in turn requires careful preparation of the printing fluid.
It is the object of the present invention to provide a radiation flood source having a homogeneous or controlled inhomogeneous radiation field. It is further an object of the invention to provide for a simple and inexpensive method of manufacture for such homogeneous radiation flood source which method further allows to omit the extensive quality testing and scanning necessary with current flood sources, and also avoids cumbersome and costly preparation of a printing fluid.